Hydraulic apparatus



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Unted States Patent O 3,142,963 HYDRAULIC APPARATUS Oswald Thoma, Charlton Kings, England, assigner to Dowty Hydraulic Units Limited, Ashcllurch, England, a British company, and Unipat A.G., Glarus, Switzerland, a Swiss company Filed (let. 16, 1962, Ser. No. 230,893 Claims priority, application Great Britain ct. 26, 1961 21 Claims. (Cl. 60--53) This invention relates to a hydraulic power transmission which comprises a variable positive displacement hydraulic pump in hydraulic connection with a variable positive displacement hydraulic motor, the pump being adapted to receive power from a power source and the motor being adapted to deliver power to a load, the speed ratio between pump speed and motor speed being determined by the relative displacements of the pump and motor.

The present invention is concerned with a hydraulic power transmission having a pump and a motor each of the tilting head kind and each comprising a rotary member or drive ange secured to a drive shaft and a head tiltable about a tilt axis relatively to the drive flange. The head comprises a frame or casing containing a rotary cylinder block, a rotary drive means between the drive ange and the cylinder block, and pistons associated with the cylinder block and arranged for reciprocation during rotation of the rotary member. The tilt angle of the head relatively to the rotation axis of the rotary member determines the effective stroke of the pistons in their cylinders. A valve means containing a pair of ports is used for the supply and the return flow of liquid to and from the cylinders. A pump or motor of the kind set forth in this paragraph will, for convenience, be referred to hereafter as a tilting head pump unit or a tilting head motor unit.

In accordance with the present invention a power transmission comprises a tilting head pump unit hydraulically connected to a tilting head motor unit, and a rigid link of invariable length, but always of less length than the spacing between the two drive anges, having one pivotal connection with the pump tilting head and another pivotal connection with the motor tilting head, with its two pivotal connections so located in relation to the length of the link and the spacing between drive anges, that tilting movement of the one titlting head causes tilting movement of the other tilting head, the link occupying a dead centre position at or near the zero piston stroke position of the pump and the motor occupying, at such dead centre position, a tilt angle corresponding substantially to the maximum motor piston stroke position permitted by the link.

The term dead centre position refers in this specification to the link position in which a straight line joining the centres of the pivotal connections, lies at right angles to the instantaneous direction of movement of the pump head pivotal connection.

The tilting heads may be adjacent and the tilt aXes may be remote from one another.

Preferably, the valve means is formed as a plate and is remotely positioned in the head unit from the rotary member or drive flange. The cylinder block is urged onto the valve plate during operation by hydraulic pressure in the cylinders. The pivotal connection between the link and each head is located adjacent to the valve plate of the head. Preferably, the pivotal connections for the links are carried by lugs extending from the valve plate in a direction away from the rotary member or drive flange.

The link may include one or more hydraulic connections between the pump and the motor, or the hydraulic connections may be independent of the link.

One embodiment of the invention will now be described with reference to the accompanying drawings in which,

FIGURES 1 and 2 are longitudinal cross-sections perpendicular to one another through the transmission, and

FIGURES 3, 4 and 5 are diagrammatic illustrations of various relative positions taken up by the pump and the motor.

The embodiment to be described is particularly intended for use as a power transmission on a vehicle such as an agricultural tractor to transmit power from the tractor engine to the ground engaging wheels. The transmission is contained within a support 1, in the form of a hollow container within which are located the pump and the motor units. The support is adapted to be secured directly to the tractor engine by means of a bell shaped housing 2. Within this bell housing the engine ilywheel 3 is conveniently located. Within the support 1 the pump unit 4 is carried by a bearing housing 5 secured to a flange 6 within the support 1. Similarly the motor unit '7 is carried by a bearing housing 8 secured to a convenient flange 9 within the support 1. For convenience in assembly and servicing of pump and motor units, the support 1 is provided with openings covered by sheet metal covers 11 and 12 secured in position by studs 13.

The tilting head pump and motor units are of substantially similar construction and the pump unit 4 only is described and illustrated in detail. The pump drive shaft 14 is carried in bearings 15 in the bearing housing 5. The inner end of the drive shaft 14 is integrally formed with a drive flange 16 which forms the rotary member of the pump. The flange 16 bears against an end surface 17 of the bearing housing 5 for the absorption of end thrust. Within the flange 16 a plurality of ball joints 13 are located whose centres lie on a circle co-axial with the drive shaft 14, the plane of which centres is always perpendicular to the shafts axis. From these ball joints connecting rods 19 extend.

Also forming part of the bearing housing 5 are a pair of lugs 21 and 22. These lugs have bearing holes 23 and 24 co-axially arranged on an axis forming a diameter of the circle including the centres of the ball joints 18. Within the bearing holes 23 and 24 the trunnions 25 and 26 of the pump head casing 27 are located, whereby the pump unit may tilt about such diameter.

A recess 28 within the casing 27 contains a rotary cylinder block 29 within which is formed a plurality of cylinders 31 located around the cylinder block at equally spaced intervals and having their axes parallel to the axis of cylinder block rotation. The cylinder block is mounted for rotation on a flat valve plate 32 which is formed as an integral part of the casing 27 at .the bottom of the recess 28. A central pin 33 extending from the valve plate 32 enters into a central bore 34 of the cylinder block to center the cylinder block for rotation on the valve plate.

Within each cylinder 31 a piston 35 is mounted for reciprocating movement. Each piston includes a deep bore 36 to receive the end of a connecting rod 19. The inner end of the connecting rod 19 is of part spherical shape 37 to it the base of the recess 36 within the piston. The connecting rod is locked within the bore 36 by means of a circlip 3S which prevents the connecting rod from leaving the bore 36 but at the same time permits slight angular movement of the connecting rod within the bore 36. The purpose of this slight angular movement is to cause the connecting rods during rotation to engage the sides of the bores 36 so that rotational drive is transmitted from the ilange 16 to the cylinder block 29. Any other means, such for example as a universal joint conneeted between the flange 16 and the cylinder block 29,

may be used to rotate the cylinder block.

The face 39 of cylinder block 29 making contact with the valve plate 32 includes a plurality of regularly spaced ports 41 which open from the face 39 one into each cylinder 31. Within the valve plate 32 a pair of kidney shaped ports 42 and 43 are provided from which passages respectively 44 and 45 extend through the wall of the casing 27 to the trunnions 25 and 26. At the trunnions these passages are arranged co-axially with the trunnions and pass through sealing members 46 and 47 to fixed connections 48 and 49 secured to the lugs 21 and 22. For the purpose of assembly the trunnions 25 and 26 are detachable from the casing 27 and are secured thereto by screws 51, and the xed connections 4S and 49 are also detachable, being secured to the lugs 21 and 22 by screws t).

It will be seen that the casing 27 together with the enclosed cylinder block 29 form the tilting head which may be tilted about a tilt axis formed by the axis of the trunnions 25 and 26. The pipe connections 4S and 49 connect the passages 44 and 45 to pipes 52 and 53 extending respectively to the corresponding pipe connections 54 and 55 of the motor unit 7.

The pump drive shaft 14 is connected to the flywheel 3 for driving by the engine.

The transmission will include customary auxiliary valves and a customary make-up pump for the purpose of ensuring that pump and motor units and the passages interconnecting them are maintained primed with hydraulic liquid. These, being conventional, are omitted in order to avoid confusion.

Assuming that the head of the pump is inclined in the trunnions so that the cylinder block axis is inclined to the drive shaft axis, rotation of the drive shaft 14 will cause rotation of the cylinder block 29 by virtue of any suitable rotary drive connection between them, such as the engagement of the connecting rods in their bores 36. Consequently the pistons 35 will be reciprocated within their cylinders and this will cause liquid ow through ports 41 and through ports 42 and 43.

The direction of this displacement of liquid will depend on the direction in which the pump head is turned about the trunnions from the position in which the cylinder block is co-axial with the flange 16. For instance assuming that liquid flows along the passage 44 into the port 42 being taken into the cylinders 31 connected with port 42, the liquid will subsequently be displaced from the cylinders into the port 43 and passage 45.

The motor 7 may thus receive liquid from either of the pipes 52 or 53 and such liquid will be fed through the ports in its valve plate to the cylinders,iwhere it will re-` act on the pistons to cause the cylinder block and the motor drive flange 56 to rotate, in turn driving the motor drive shaft 57. The motor drive shaft 57 is carried within bearings 58 in the bearing housing 8 in a similar manner to the drive shaft 14 of the pump unit. Also the hearing housing 8 includes lugs 59 and 61 which carry trunnions 62 and 63 of the motor head' unit in the manner similar to that described for the pump unit. It will be noted that the tilting heads of pump and motor are adjacent and that the trunnions are remote fromeach other.

The head casing 64 of the motor and the head casing 27 of the pump are mechanically interconnected by means of a rigid link 65 having pivotalconnections to the pump and motor headV casings. The pivotal connection to the motor head casing 64 is provided by means of a bifurcated lug 66 extending from the valve plate part of the casing 64 away from the rotary member 56 and by means of a pivot pin 67 adapted to extend through the lug 66 and Ithrough a hole 68 in one end of the link 65. The pivotal connection on lthe pump head casing 27 comprises a bifurcated lug 69 adapted 4to receive the other end of the link 65 and through which a pivot pin 71 extends, passing through a hole 72 in the end of the rigid link 65.

The bifurcated lug 69 is also of sufficient size to accommodate a driving link 73 which engages the same pivot pin 71 between the bifurcations. The link 73 is carried by the driving rod 74 at pivotal connection 75, the rod 74 extending from a servo motor 76 mounted in the support 1. The servo motor 76 may take any well known form or alternatively it may take the form disclosed in United States Let-ters Patent No. 3,017,750.

A control rod 77 for the servo motor extends from the upper end thereof and movement of this rod into and out of the servo motor 76 will cause corresponding movement with considerably increased force of the rod 74. Stops (not shown) may be provided to limit the movement of the moving part of the servo motor with respect to the fixed part.

The length of the link is arranged to be considerably shorter than the spacing between the drive flanges 16 and 56 of the pump and motor thus permitting a construction of transmission in which the drive shaft 14 of the pump and the driven shaft 57 of the motor extend oppositely away from the support 1. The length of the link 65 together with the location of the pivot pins 67 and 71 are so arranged and interrelated that in or near the zero piston stroke position of the pump 4 the axes of the pivot pins 67 and 71 are co-planar with the trunnion axis of the pump head 27 to give the dead centre position. In other words, and referring to FIGURE 3, a straight line 81 passed through the axes of 67 and 71 will also pass through the axis 22a of trunnions 25 and 26. In this position the instantaneous direction of movement of the pivot pin 71 attached to the pump is at right angles to the plane extending through the axes of the pivot pins 67 and 71. With such a dead centre position the axis of the pin 67 is displaced the maximum possible distance from the axis of the trunnions 25 and 26 so that the motor head 64 is at its maximum possible angular displacement. Thus, movement of the pump head 27 in either direction from the zero piston stroke position will cause a reduction in the tilt angle of the motor head 64 and thus reduction in motor piston stroke. It should be noted that the direction of movement of servo rod 74 is generally perpendicular to the line 81, hence the movement of pump head 27 in either direction is strongly controlled by the servomotor 76.

By arranging that the servo motor 76 acts directly on the pump head unit it is possible by continuous movement of the servo motor to move from maximum angular displacement of the pump headin one direction through zero displacement to maximum angular displacement in the opposite direction, during which movement the motor head 64 will move from the minimum angular position permitted by link 65 to the maximum angular displacement position permitted by link 65 and back again to another minimum displacement position permitted by link 65.

Referring to FIGURE 3 the pump is in the zero displacement position and the link 65 is in the dead centre position, thus maintaining the motor at its maximum displacement. ln this dead centre position the pivot axis of pivots 67 and 71 of the link and the axis of pump trunnions 25, 26 are aligned as'indicated by the dotted line 81. The instantaneous .direction of movement of the pump pivot 71 is then at right angles to the line joining the axes of pivots 67 and 71. In FIGURE 4 the'pump head has moved to its maximum displacement position in the forward direction and the link 65 has moved the motor head 64 to a minimum displacement position. This relative position of pump and motor gives the maximum motor speed in the forward direction of propulsion. In FIGURE 5 the pump head has moved through its neutral or zero displacement position to its opposite maximum displaceemnt position and the link 65 has moved the motor head again to a minimum displacement position thus giving the maximum motor speed in the reverse direction. The minimum displacement position of the motor head in the forward direction is different from that of .the motor head in the opposite direction so that the maximum motor speed in the forward direction is greater thanthe maximum motor speed in the opposite direction.

Whilst the dead centre arrangement of the link 65 gives the advantage that it permits reduction of motor displacement both for forward and reverse displacement of the pump it also has a further and less obvious advantage. With a variable displacement pump as described the pump, on approaching zero displacement, is capable of generating very substantial pressures in the pumped liquid, and such pressures must necessarily be limited by a high pressure relief valve to values at which the various parts of the transmission are not overstressed. This means that for a small displacement range of the pump head on either side of the Zero displacement position (in which latter position the pump pistons have no stroke) the pressure cannot exceed a certain maximum value. This in turn means that during acceleration of the motor under load from rest the maximum torque which the motor is able to exert will be dependent solely on the displacement of the motor head 64 since the pressure cannot exceed the relief valve setting. Any substantial reduction of motor displacement during such initial acceleration of the motor would reduce the available torque and thus reduce the acceleration. The dead centre arrangement of the link 65 has the further advantage that in movement of the pump head from the zero displacement position in either direction reduction of motor displacement will be very slight for a substantial alteration in pump displacement. Thus for a substantial change in pump displacement from the zero position the motor is capable of exerting a maximum torque. Only after the pump has assumed a substantial displacement whilst the link 65 begins effectively to reduce motor displacement does the motor torque reduce substantially. Consequently, although the illustrated embodiment is of a reversible transmission, the invention can, with advantage, be applied to a non-reversible transmission.

In the illustrated embodiment the link 65 is of solid construction and the hydraulic liquid passes between pump and motor through the pipes 52 and 53. It is however within the scope of the invention that the link 65 may accommodate one or two hydraulic interconnections between the pump and the motor. For this purpose the pivotal connections 67, 68 and 71, 72 will be formed as hydraulic pivotal couplings to supply hydraulic liquid to one or more passages withiri the link 65. An example of an interconnection of this type is shown in the copending application of Dowty and Andrews, Serial No. 289,601, filed June 21,1963.

A further advantage of the embodiment described lies in the fact that the pump and motor units are both such that the effect of hydraulic pressure within them is to cause the tilting head to tend to move to the zero displacement position. This arrangement of the link 65 is such that whatever the selected speed ratio of the transmission the pump and motor heads move differentially, i.e. increase in displacement of the pump head results in reduction in displacement of the motor head and vice versa. This will mean that the self restoring forces acting on the tilting heads will oppose one another through the link 65 whereby the servo motor 76 need only supply a force for adjustment of the heads which is a function of the diiterence of the self restoring forces acting within the heads.

The illustrated embodiment is only one of many possible constructions within the scope of the present invention. For example, the construction of the tilting head pumps or motors may be considerably varied from that illustrated whilst still retaining a head unit including a valve and a rotary cylinder block. It is not necessary that the pump should be of the reversible displacement type and it is within the scope of the invention to provide a pump adjustable in one direction only from zero displacement and acting through a dead centre linkage to control motor displacement. It is further not necessary to arrange that the trunnion axes of pump and motor and the pivot pins of the link 65 are arranged parallel to one another. It is within the scope of this invention to arrange that the trunnion axes of pump'and motor are in noncoaxial relation one to the other, being relatively parallel, inclined, or skewed, as in the copending application of Thoma and Ward, Serial No. 255,256, tiled January 31, 1963, and wherein the interconnecting link 65 may act through pivotal joints of the ball type in between the pump and motor heads.

In the described embodiment of this invention provision may be made for a power take-oftr connection from the engine which comprises a pair of gears 81 and 82 connected respectively to the pump drive shaft 14 and to a power take-off shaft 83 carried in bearings 84 and 85 of the support 1. By adjustment of the selected displacement of the pump head to the zero position it is possible to supply power from the engine to the power take-off shaft 83 without propelling the vehicle. The power takeoi shaft 83 may conveniently pass through and form the drive shaft for a small gear pump 86 which serves the purpose of supplying the transmission with liquid at low pressure to maintain it primed with liquid and at the same time to supply hydraulic power for operation of the servo motor 76.

I claim as my invention:

l. A power transmission comprising a tilting head pump unit pivotable about a given tilt axis between positions of zero and maximum displacement, means so to tilt said pump unit, a tilting head motor unit pivotable about a different tilt axis, at a given spacing from the tilt axis of the pump unit, between positions of maximum and minimum displacement, conduit means interconnecting said units for transfer of hydraulic luid therebetween, to drive the motor unit from the pump unit, and a rigid link of unvarying length connected at a rst pivot point to the motor unit, and at a second pivot point to the pump unit, the length of said link and the locations of the two pivot points being such, in relation to the given spacing between the tilt axes, that at or near the zero displacement position of the pump unit the motor unit is disposed in its maximum displacement position, and a straight line joining the centres of the two pivot points of the link lies at right angles to the instantaneous direction of movement of the pump units pivotal connection to the link, whereby movement of the pump unit from such position, at or near zero displacement, will tilt the motor unit to a position of lesser displacement.

2. A power transmission as in claim 1, wherein a straight line which passes through the two pivot axes of the link also passes through the tilt axis of the pump unit, when parts are at or near zero displacement position of the pump unit, as speciied in claim l, but not in any any other position.

3. A power transmission as in claim l, wherein the tilting heads of the two units are adjacent and their respective tilt axes are remote each with respect to the other.

4. A power transmission as in claim 1, wherein the pivot points of the link upon the respective tilting heads are at positions remote from the tilt axes of the respective units, and at a spacing less than the spacing between such tilt axes.

5. A power transmission as in claim 4, wherein the pivot points of the link upon the respective tilting heads are oitset, to the same side, from their respective axes of rotation, and wherein the spacing of said pivot points is such in relation to the spacing between the tilting axes of the respective heads, that when the pump unit is sub-` stantially in its position of Zero displacement the motor unit is tilted, by the disposition of the links pivot points in line with the pump units tilting axis, to a position of maximum displacement, such that tilting of the pump units head by given increments in the direction from such position to aiTord forward drive, tilts the motor units head by corresponding increments to a position of lesser displacement, but tilting of the pump units head in the opposite direction by the same increments from such position, to aitord reverse drive, tilts the motor units head also to a position of lesser displacement, but by greater increments in relation to its forward drive increments.

6. A power transmission as in claim 4, wherein the pivot points of the link upon the respective tilting heads are offset, to the same side, from their respective axes of rotation, and wherein the spacing of said pivot points is such in relation to the spacing between the respective tilting axes, that when the pump unit is substantially in its position of zero displacement the motor unit is tilted, by the disposition of the links pivot points in line with the pump units tilting axis, to a position of maximum displacement, whereby tilting of the pump unit in either direction from that position toward its position of maximum displacement tilts the motor unit towards a position of lesser displacement.

7. A power transmission as in claim 1, wherein the pump unit is of the reversible displacement type, and is tiltable to that end by continuous angular movement through and beyond its position of zero displacement.

8. A power transmission as in claim 1, wherein the means to tilt the pump unit is arranged for movement in a line which, at or near the position of zero displacement, is perpendicular to the straight line which passes through the tilt axis of the pump unit and the two pivot points of the link.

9. A power transmission as in claim 1, wherein the tilt axes of the two units are parallel, and the axes of the links pivot points are also parallel, and parallel to the tilt axes of the two units, whereby when the pump unit is at or near its position of zero displacement the two pivot axes of the link and the tilt axis of the pump unit lie in a common plane.

10. A power transmission as in claim 9, wherein the tirst pivot point of the link is olfset laterally of the rotational axis of the motor unit, and the second pivot point of the link is olfset laterally to the same side, but with respect to the rotational axis of the pump unit.

l1. A power transmission comprising a tilting head pump unit pivotable about a given tilt axis to both sides of a neutral position. of zero displacement, for drive in the forward or reverse directions, means so to tilt said pump unit, a tilting head motor unit pivotable about a tilt axis remote from and parallel to the tilt axis of the pump unit, said units being disposed with their tilting heads adjacent and their tilt axes remote, conduit means interconnecting said units for transfer of hydraulic iluid therebetween, to drive the motor unit in the forward or the reverse direction from the pump unit, and a rigid link of unvarying length connected to the tilting head of the pump unit at a rst pivot point, intermediate the two tilt axes, on an axis parallel to the two tilt axes and offset to one side of a plane common thereto, and connected also to the tilting head of the motor unit at a second pivot point, also intermediate the two tilt axes and on an axis parallel to the other axes, and offset to the same side of the plane common to the two tilt axes, the length of the link being such in relation to the locations of its two pivot axes that when the tilting head of the pump unit passes.

to either side of its neutral position the tilting head of the motor unit remains always tilted to the same side of the plane common to the two tilt axes, and moves from a position of'maximum displacement corresponding substantially to the pump units position of zero displacement, toward a position of minimum displacement correspending substantially to the pump units position of maximum displacement.

12. A power transmission comprising a tilting-head pump-unit and a tilting-head motor-unit, each including a rotative drive flange, means hydraulically interconnecting said pump-unit and said motor-unit, a rigid link of unvarying length having a pivotal connection at one end With the pump tilting-head and another pivotal connec- 8 tion at its other end with the motor tilting-head, so that tilting movement of one tilting-head causes tilting movement of the other tilting-head, the spacing between the pivotal connections of the link being less than the spacing between the respective drive flanges.

13. A power transmission as in claim 12, characterized in that drive shafts extend from the respective units in the direction away from the other unit, and the respective tilting-heads are adjacent one another and their tilting axes are remote.

14. A power transmission as in claim 13, wherein the two drive shafts are disposed coaxially.

15. A power transmission comprising a tilting-head pump-unit and a tilting-head motor-unit, means mounting each thereof for tilting about its individual tilting axis, and each including a rotative drive llange located to include the corresponding tilting axis, disposed diametrally of said drive flange, each tilting head extending from its drive flange towards the tilting axis of the other unit, means hydraulically interconnecting said pump-unit and said motor-unit, a rigidV link of unvarying length pivotally connected at one end to the pump tilting-head, and pivotally connected at its opposite end at a fixed distance less than the spacing between the two drive anges, to the motor tilting-head, whereby tilting movement of one tilting-head causes tilting movement of the other tiltinghead.

16. A power transmission as in claim 15, wherein the tilting axes of the pump-unit and of the motor-unit are disposed parallel one to the other.

17. A power transmission comprising a tilting-head pump-unit including a rotary flange, a tilting-head pivotally mounted to tilt about an axis intersecting the rotary axis of said ange, pistons reciprocable within cylinders disposed in said tilting-head about the rotational axis, and connecting rods each articulated wth respect to the rotary flange and to its respective one of said pistons, a tiltinghead motor-unit including a rotary flange, pistons, and connecting rods similarly related to those of the pumpunit, and a tilting-head pivotally mounted to tilt about an axis intersecting the rotary axis of its ange, the two tilting heads being arranged each adjacent the other, with their tilting axes remote, means hydraulically interconnecting said pump-unit and said motor-unit, a rigid link of xed length pivotally connected at one end to the tilting-head of said pump-unit and at its other end to the tilting-head of said motor-unit, both said pivotal connections being olset from the respective tilting axes to the same side of the respective rotational axes, whereby when the pump-units tilting-head is tilted from a position at or near its zero delivery position, coincident with the axis of its rotary flange, the motor-units tilting-head is tilted with respect to the axis of its rotary ilange towards a position of lesser piston stroke.

18. A power transmission comprising a tilting head pump unit having a driving shaft protruding at one end and a tilting head tiltable about a tilting axis, transverse to and intersecting the axis of its driving shaft between a position of zero displacement, wherein the head is coaxial with the driving shaft, and a position of maximum displacement, means so to tilt the head of said pump unit, a tilting head motor unit likewise having a protruding driving shaft, coaxial with the pump units driving shaft, and a tilting head tiltable about a similarly related transverse tilting axis, between positions of maximum and minimum displacement, conduit means interconnecting said units for transfer of hydraulic fluid therebetween, to drive the motor unit from the pump unit, and a rigid link of unvarying length connected at a lirst pivot point to the tilting head of the motor unit, and at a second pivot point to the tilting head of the pump unit, the length of the link being such, in relation to the spacing between the tilt axes, and the two pivot points being so located on the respective units, that when the pump unit is substantially in its position of zero displacement the motor unit is tilted to a position of maximum displacement, and a line joining the two pivots of the link passes also through the tilting axis of the pump heads tilting axis, whereas at the pump units position of maximum displacement the motor unit is tilted to a position of minimum displacement.

19. A power transmission as in claim 18, wherein the tilting heads of the pump and motor units are located each adjacent the other, with their tilt axes remote, and their respective driving shafts protrude in respectively opposite directions, away from one another.

20. A power transmission comprising a tilting head pump unit having a driving shaft the axis of rotation whereof is fixed, and a tilting head tiltable about a tilting axis transverse to and intersecting the rotational axis of its driving shaft, between a position of zero displacement wherein the rotational axis of the head is coaxial with the driving shaft, and a position of maximum displacement, means so to tilt the head of said pump unit, a tilting head motor unit likewise having a driving shaft the axis of rotation whereof is fixed, a tilting head tiltable about a transverse tilting axis similarly related to the rotational axis of its driving shaft, between positions of maximum and minimum displacement, which tilting axis is spaced by a deiinite distance from the tilting axis of the pump unit, conduit means interconnecting said units for transfer of hydraulic fluid therebetween, to drive the motor unit from the pump unit, and a rigid link of unvarying length connected at a rst pivot point to the tilting head of the motor unit, and at a second pivot point to the tilting head of the pump unit, the length of said link being such with respect to the spacing of the tilting axes, and its two pivot points being so located upon the respective units, that when the pump unit is substantially in its position of zero displacement the motor unit is tilted to a position of maximum displacement, and a line joining the 1t) two pivots of the link includes also the tilting axis of the pump head, whereas at the pump heads position of maximum displacement the motor units head is tilted to a position of minimum displacement.

21. A power transmission comprising a rst and a second tilting head unit, the rst tilting head being tiltable through a predetermined angular range, means hydraulically interconnecting said units for driving one unit from the other, an adjusting means positively to adjust the angular setting of the irst head between the predetermined limits, a rigid link of unvarying length having one pivotal connection with the rst tilting head and another pivotal connection with the second tilting head, the length of the link and its arrangement relative to the two tilting heads being such that within the predetermined angular range of the first head it enters a dead center position in which a straight line joining the centers of the pivotal connection with the second tilting head, the length direction of movement of the irst head pivotal connection, but is incapable of entering a position in which a straight line joining the centers of the pivotal connections lies at right angles to the instantaneous direction of movement of the second head pivotal connection, whereby the adjusting means acting upon the rst head directly will adjust the same through the dead center position to either side thereof, but acting upon the second head indirectly, through the link, will adjust said second head throughout a range corresponding to the range of adjustment of the rst head.

References Cited in the tile of this patent UNITED STATES PATENTS 2,956,407 Grabow Oct. 18, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noe 3,142v93 August 4, 1964 Oswald Thoma It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should -read as corrected below.

Column lO, lines 18 and 19, strike out "pivotal connection with the second tilting head, the length direction of movement of the first head pivotal Conneo-" and insert instead pivotal connections lies at right' angles to the instantaneous direction of movement of the first head pivotal connec- Signed and sealed this 24th day of November 196417 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitesting Officer Commissioner of Patents 

12. A POWER TRANSMISSION COMPRISING A TILTING-HEAD PUMP-UNIT AND A TILTING-HEAD MOTOR-UNIT, EACH INCLUDING A ROTATIVE DRIVE FLANGE, MEANS HYDRAULICALLY INTERCONNECTING SAID PUMP-UNIT AND SAID MOTOR-UNIT, A RIGID LINK OF UNVARYING LENGTH HAVING A PIVOTAL CONNECTION AT ONE END WITH THE PUMP TILTING-HEAD AND ANOTHER PIVOTAL CONNECTION AT ITS OTHER END WITH THE MOTOR TILTING-HEAD, SO THAT TILTING MOVEMENT OF ONE TILTING-HEAD CAUSES TILTING MOVEMENT OF THE OTHER TILTING-HEAD, THE SPACING BETWEEN THE PIVOTAL CONNECTIONS OF THE LINK BEING LESS THAN THE SPACING BETWEEN THE RESPECTIVE DRIVE FLANGES. 